1. Field of the Invention
The invention relates to an improved process for the preparation of dialkyl carbonates by oxycarbonylating the corresponding alkanols in the presence of Cu salts, the water content being limited to less than 10 wt % of the reaction mixture.
Dialkyl carbonates, especially dimethyl carbonate, are intermediates of low toxicity and can replace toxic intermediates, such as phosgene or dimethyl sulphate, in many reactions. They are also non-corrosive. Their use does not give rise to environmentally harmful byproducts.
Examples of such reactions of dialkyl carbonates are the preparation of urethanes from aliphatic or aromatic amines, which can in turn be cleaved to give the corresponding isocyanates. As another example, dimethyl carbonate can replace dimethyl sulphate in the quaternization of amines or in the methylation of phenol or naphthols. Dimethyl carbonate can also be added to motor fuel, e.g. instead of lead compounds, to improve the octane number. Despite this importance of dialkyl carbonates, there is still no technically simple and environmentally acceptable production process which is suitable for large capacities without substantial byproduct formation or coupled material cycles.
2. Description of the Related Art
There are various processes, already technically proven on a small scale, for the preparation of dialkyl carbonates. The methods of preparation which are based on the catalytic reaction of alkanols with carbon monoxide and oxygen according to the equation below have been the subject of intensive study by a variety of working groups: ##STR1##
In these reactions, the catalytically active copper compounds have been used in the form of various copper salts. When using copper(II) chloride as the catalyst according to JP-45/11129 (1970), unsatisfactory selectivities are obtained. Particularly troublesome is the formation of relatively large amounts of methyl chloride, which, because of its high volatility, tends to spread ubiquitously throughout the whole of the production plant and in practice can cause corrosion in the entire plant.
Better selectivities are obtained when using organic complexing agents (DE-A 21 10 194), but then there is the problem of separating off the catalyst salts, which are partly dissolved in the reaction mixture but are largely present as a suspension.
Carrying out this reaction according to DE-A 27 43 690 presents a very particular problem because the catalyst salts are practically completely undissolved in the reaction mixture and are only present as a suspension. These salts have to be conveyed through the reaction zone and the cooling units and separated off mechanically after the reaction, e.g. by means of centrifuges. In addition to the corrosion already mentioned, this also causes erosion, poor heat transfer, clogging and encrustation.
To avoid these disadvantages of a catalyst cycle, it has been proposed to keep the catalyst salts stationary in the reactor as a suspension and to meter methanol, CO and oxygen into the reactor, the dialkyl carbonate formed and the water of reaction being distilled out of the reactor together with the methanol used in excess (EP 0 413 215 A2). Here the liquid reaction medium consists essentially of the alkanol to be reacted (EP-0 413 215, page 3, line 52), so the molar ratio of alkanol to Cu salt is very high (preferably 1:0.01-0.05). This has the disadvantage of a relatively low reaction rate. A further problem here is the necessity to establish a low dialkyl carbonate concentration.
This is not easy since the reaction is carried out at a high system pressure and both dialkyl carbonate and water are very soluble in the reaction medium, which consists essentially of methanol. This means that a relatively large amount of inert gas or methanol gas has to be used to force the dialkyl carbonate and water to separate out.
Moreover, with this proposal, it must also be taken into account that the catalyst has to be changed after some time or continuously renewed in a certain proportion, which entails the problem, mentioned at the outset, of the separation, regeneration and recycling of the catalyst.